1. Field of the Invention
An object of the present invention is a gamma camera, preferably with two opposite detectors having independent radial movements. In this camera, the ergonomy of use as well as the efficiency of detection are improved, since the detectors are maintained independently, at the smallest possible distance from the patient to be examined, thus increasing the resolution.
2. Description of the Prior Art
Gamma cameras are described, for example, in the U.S. patent by Anger, U.S. Pat. No. 3,011,057. A gamma camera is an apparatus comprising a stand, which is fixed or rotating or even movable with respect to the ground and carries a detector, also called a detector head, at the end of an arm. This detector is provided with an array of photomultiplier tubes, the input faces of which are juxtaposed with one another and constitute the detection surface of the detector head and its detection field.
The following is the principle of the examination. A radioactive substance is injected into a patient to be examined. This substance is thallium for example. The radioactive emission excites a scintillator crystal of the detector which converts the energy of the gamma photons into a light energy that can be detected by the photomultiplier tubes. The scintillator crystal is preceded by a collimator.
The scintillations emitted are detected by the photomultiplier tubes which produce electrical signals depending on the light intensity received. By carrying out barycentric tracking operations on all these electrical signals, it is possible, in a known way, to determine the localization X Y of the origin of the scintillation in the detection field. An incremental acquisition is then carried out by totalizing the number of scintillations (or strokes) detected per localization element called a pixel.
By leaving the detector head in a given position for a certain time above the examined body, it is then possible, for a given angle of sight, called a projection, to obtain an image that reveals the concentration of the emitting substance in the body. A tomographic examination consists in acquiring one image per angle of sight, for a large number of angles of sight, evenly spaced out on an angular sector of at least 180.degree.. It is then possible, with computation algorithms, notably filtered back projection, to reconstitute the image of a volume of the body.
To increase the sensitivity of the camera, the common practice has been to use a stand, capable of rotating if necessary and provided with two detector head instead of only one. These two heads face each other. When they rotate, the heads rotate together about the patient being examined.
The stand is normally capable of making a rotation on itself about a tomographic axis of rotation. The detector head is held by an arm fixed to the stand by a mechanism of radial movement. The arm that supports the head describes a cylinder in the course of the rotation. In principle, the detector head is oriented so that the normal to its detector field, in the center of this field, is perpendicular to the rotation axis of the stand. When a radial movement is carried out, the arm is moved away or brought closer to this rotation axis. When the gamma camera concerned is one with two detector heads, each of these heads is mounted on an arm. The two arms are fixed symmetrically to the stand, on either side of the rotation axis. Besides, they are shifted symmetrically about this axis, both in rotation and in radial movement.
In a tomographic examination, the two heads rotate about the patient and the value of the radius, for each angle of rotation, should be chosen in such a way that none of the detectors comes into contact with the patient. The two radii of spacing of the heads are identical. Since the patient has no symmetry of revolution, there is always one detector that is further away from the patient than the other one. For this detector which is further away, there is a loss of resolution. The resolution is poorer than if this detector were to be as close as possible to the patient.
Furthermore, when the examination covers the whole body, the patient is placed on a bed and the stand bearing the two facing detectors is driven by a lateral movement. Given the symmetrical radial movement of the above-mentioned heads, it being known, firstly, that the lower head should keep an altitude, or a radius value, that is constant to change position at a fixed distance from the bed and, furthermore, it being known that the upper head should provide a passage for the patient's most prominent parts (generally his belly), it is necessary to fix the value of the radius throughout the lateral movement, thus penalizing the resolution of the upper head.